I don't actually write them all exactly as MC spits them out. I do transform some to more flac/Vorbis standard (e.g. "Track #" into "Tracknumber" or "Name" into "Title") but there's only a couple. Same thing with the Wavpack plugin and the APE tags.
Yes, I know. I simplified the reply a bit. Naturally JRiver does similar things with the commonly used basic tags.
I think you do something also with the FLAC date/year values. Actually, I may have noticed some strange things with the "year" behavior, but that's OT here. (I'll report in the plugin forum if I stumble on that again.).
As you may remember, I criticized the tagging behavior of your Wavpack plugin first, but I have changed my mind.
In general, I don't think that users like me who have 50000 already tagged files in an MC library care much how the custom tags are saved to the files. All basic tags are saved in a common format and they work with other programs and devices.
However, If I happen to buy a new device which supports e.g. the mentioned "Composer" tag I would like to be able to add that tag with MC.
A real life example is the Rockbox firmware (for iPods etc). It supports the "ALBUM ARTIST" Ogg Vorbis field for grouping multiple artists albums, but unfortunately MC writes that field as "MEDIA JUKEBOX:ALBUM ARTIST".
Most useful the possiblility to easily link a custom Vorbis comment or APE tag field with a similarly named library field would be for MC newcomers if they happen to have a lot of custom tags that are created with other programs.
A few years ago most other programs supported only a few common ID3v1 and v2 tags. MJ was one of the rare programs that allowed proprietary custom tags. Back then it was natural to follow the ID3v2 comment system with other file formats and use a prefix for identifying the creator of the custom tags.
The situation has changed since then. All other programs that don't limit tagging of these formats allow writing new custom tags with the actual visible names. The Vorbis comments and APE tags don't have the ID3v2 limitations and the "Media Jukebox:" prefix is redundant and limiting.
EDIT
Matt, what is the story behind the Preference tag? I tested several programs, but couldn't find any that uses it.